Shamima Begum, who joined the 15-year Islamic State Group in Syria, loses her British citizenship.
The lawyer of the family Tasnime Akunjee, said they were "disappointed" with the decision and considered "all legal means" to challenge them.
Ms. Begum left Bethnal Green in East London in February 2015 with two school friends.
She was found 19 in a Syrian refugee camp after leaving Baghuz, the IS's last fortress.
BBC interview, Ms. Begum, who has just been born, had said that she now wants the forgiveness of Great Britain and wants to return home.
- "I did not want to be an IS poster girl"
- IS-Teenage Bride Begum Bears  ITV News said that Ms. Begum's mother had received a letter from the Home Office on Tuesday.
The letter states that Interior Minister Sajid Javid ordered the move and had already been processed.
He asked the mother to inform her daughter of the decision and to inform her that she had a legal remedy to appeal. Whitehall sources confirmed that the Home Secretary had made the decision. However, it is not clear why this was done.
According to the British Nationality Act of 1981, someone may be deprived of citizenship if the Minister of the Interior is convinced that he is "conducive to the public good" and thus not become stateless.
Ms. Begum said that she traveled to Syria with her sister's British passport, but was taken away from her when she crossed the border.
It is believed that she is from Bangladesh, but when asked by the BBC, she said she had no passport in Bangladesh and had never been to the country] Ms. Begum was 15 years old and lived in Bethnal Green, London, when she left the United Kingdom in 2015
Mr. Javid told MPs on Monday that he would not "delay" the return of the British who have traveled to Syria to join the IS.
He said more than 100 dual nationals had already lost their British citizenship when traveling to support terrorist groups.